### HubbleSite - Reference Desk - FAQs

Jun 7, Reading time (words). The frequency of a waveform is the number of times it repeats (cycles) in one second. A sine wave of Hz (many. Another unit for frequency is the Hertz (abbreviated Hz) where 1 Hz is equivalent to 1 cycle/second. The period of a wave is the time for a particle on a medium to make one complete vibrational cycle. Do you observe the relationship?. Frequency means oscillations (cycles) per second in Hz = hertz = 1/s. λ = c / f = wave speed c (m/s) / frequency f (Hz). The unit hertz (Hz) was once called cps = cycles per second.

For example, it is not uncommon to hear a question like "How frequently do you mow the lawn during the summer months? If a coil of slinky makes 2 vibrational cycles in one second, then the frequency is 2 Hz. If a coil of slinky makes 3 vibrational cycles in one second, then the frequency is 3 Hz. The quantity frequency is often confused with the quantity period. Period refers to the time that it takes to do something. When an event occurs repeatedly, then we say that the event is periodic and refer to the time for the event to repeat itself as the period.

The period of a wave is the time for a particle on a medium to make one complete vibrational cycle. Period, being a time, is measured in units of time such as seconds, hours, days or years.

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The period of orbit for the Earth around the Sun is approximately days; it takes days for the Earth to complete a cycle. The period of a typical class at a high school might be 55 minutes; every 55 minutes a class cycle begins 50 minutes for class and 5 minutes for passing time means that a class begins every 55 minutes.

## Frequency and Period of a Wave

The period for the minute hand on a clock is seconds 60 minutes ; it takes the minute hand seconds to complete one cycle around the clock. Frequency and period are distinctly different, yet related, quantities. Frequency refers to how often something happens. Period refers to the time it takes something to happen. Frequency is a rate quantity.

Period is a time quantity. As an example of the distinction and the relatedness of frequency and period, consider a woodpecker that drums upon a tree at a periodic rate. If the woodpecker drums upon a tree 2 times in one second, then the frequency is 2 Hz.

Each drum must endure for one-half a second, so the period is 0.

### Frequency and Period of a Wave

If the woodpecker drums upon a tree 4 times in one second, then the frequency is 4 Hz; each drum must endure for one-fourth a second, so the period is 0. If the woodpecker drums upon a tree 5 times in one second, then the frequency is 5 Hz; each drum must endure for one-fifth a second, so the period is 0. Do you observe the relationship?

Mathematically, the period is the reciprocal of the frequency and vice versa. When the unknown frequency is applied to the electromagnet, the reed which is resonant at that frequency will vibrate with large amplitude, visible next to the scale. Stroboscope[ edit ] An older method of measuring the frequency of rotating or vibrating objects is to use a stroboscope.

This is an intense repetitively flashing light strobe light whose frequency can be adjusted with a calibrated timing circuit. The strobe light is pointed at the rotating object and the frequency adjusted up and down.

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When the frequency of the strobe equals the frequency of the rotating or vibrating object, the object completes one cycle of oscillation and returns to its original position between the flashes of light, so when illuminated by the strobe the object appears stationary. Then the frequency can be read from the calibrated readout on the stroboscope. A downside of this method is that an object rotating at an integral multiple of the strobing frequency will also appear stationary.

Frequency counter Modern frequency counter Higher frequencies are usually measured with a frequency counter. This is an electronic instrument which measures the frequency of an applied repetitive electronic signal and displays the result in hertz on a digital display.

It uses digital logic to count the number of cycles during a time interval established by a precision quartz time base. Cyclic processes that are not electrical in nature, such as the rotation rate of a shaft, mechanical vibrations, or sound wavescan be converted to a repetitive electronic signal by transducers and the signal applied to a frequency counter.